This invention concerns a method of manufacturing a transducer having a diaphragm with a predetermined tension such as a microphone. Most microphones have a diaphragm which is caused to move by the sound pressure such as microphones with electrodynamic, piezoelectric, piezoresistive, or capacitive readout. The method of the invention applies to all such types of transducers having a diaphragm.
In particular, a condenser microphone has as its basic components a diaphragm or membrane mounted in close proximity of a back plate. The diaphragm is retained along its periphery and can move or deflect in response to a sound pressure acting on a surface of the diaphragm. Together the diaphragm and the back plate form an electric capacitor, and when the diaphragm is deflected due to the sound pressure, the capacitance of the capacitor will vary. In use the capacitor will be charged with an electric charge corresponding to a DC voltage, and when the capacitance varies in response to the varying sound pressure, an electric AC voltage corresponding to the varying sound pressure will be superimposed on the DC voltage. This AC voltage is used as the output signal from the microphone.
A diaphragm with a low tension is xe2x80x9csoftxe2x80x9d and will deflect more than a diaphragm with a high tension, resulting in a higher sensitivity, which is desirable. The diaphragm of a microphone of the type considered should therefore have a well defined low tension.
Micromachined microphones have been developed by different research laboratories with applications such as in the telecommunication and hearing industry markets. One of the most challenging problems in the design and manufacturing of micromachined microphones is the controlled low tension of the diaphragm. Different sound detection principles have been suggested such as capacitive, piezoelectric, piezoresistive, optical, and tunneling read out. Most of which require a diaphragm with a tension below 50 N/m. In particular, battery-operated capacitive microphones with a low bias voltage of a few volts require very accurate control of the stress level in the diaphragm.
Conventionally, a diaphragm is glued to a metal frame using weights at the rim of the frame to adjust the tension of the diaphragm. This technique is not applicable to micromachining technology.
In micro-technology the tension of the diaphragm can be adjusted by developing new materials (e.g. silicon-rich silicon nitride), new deposition techniques (e.g. Plasma-Enhanced Chemical Vapor Deposition), new deposition conditions (e.g. by varying the temperature in a Low Pressure Chemical Vapor Deposition furnace), or subsequent temperature treatments (annealing treatments). Also the suspension of the diaphragm can relax tension e.g. through corrugations, hinges, springs, or in the most extreme case by suspending a plate.
However, the techniques currently used in micro-technology are either not reproducible and controllable enough for microphones in the above mentioned applications, or they impose other technological difficulties such as bending of suspensions and diaphragm due to a stress profile/gradient in the diaphragm.
Sensors and Actuators A. 31, 1992, 90-96 describes a transducer with a composite membrane consisting of two layers having compressive and tensile internal stress, respectively. It is described that by varying the relative thickness of the layers, the resulting internal stress can be controlled, but no method or means for doing so is disclosed.
This invention proposes a new method which can be used to tune the diaphragm stress to a predetermined level during or after processing of a micromachined microphone.
The diaphragm of the microphone resulting from the process of this invention is a sandwich of two or more layers (multi-layer, laminate, or composite) deposited on a rigid or stiff substrate. The diaphragm is formed by etching a hole into the substrate leaving the multi-layer as the diaphragm across the etched hole. In general, the layers of the diaphragm have different stress levels such as a layer of compressively stressed material and a layer of tensile stressed material, but the layers can both have compressive stress or tensile stress. This allows to achieve a desired tension level (tension=stress*thickness) by choosing the right ratio of the thicknesses of these materials. A thicker tensile layer will shift the total tension of the diaphragm to more tension, while a thicker compressively stressed material will shift the stress to more compression.
By adjusting the thickness ratio of the layers by the method according to the invention the tension can be controlled much more accurately than by any other attempt to achieve a certain stress or tension level, because thickness can be controlled almost down to the atomic level in micro-technology. It allows to deposit layers in a stable regime, where the materials have little variations in their mechanical properties. The correct stress level is adjusted by choosing the correct mixture of materials rather than the correct materials properties. Furthermore, the total thickness of the diaphragm can be chosen independently of the stress/tension level.
The total stress can be changed after deposition of the layers by changing the thickness of one or both of the outer layers. This can be done by known methods such as dry or wet etching to remove material from the outer layers, or by deposition/absorption of material to achieve thicker outer layers. Deposition on or etching of the outer layers will change the ratio of thickness. The stress or tension level of the composite diaphragm will thereby change. Etching processes can be wet etching processes using reactants such as HF, phosphoric acid, KOH, etc. or dry etching processes such as Reactive Ion Etching. Low etching rates can easily be achieved to support a controlled, accurate, and uniform removal of material. Deposition processes for tuning include physical and chemical vapor deposition.
The processes used for batch manufacturing of transducers according to the invention are very accurate and reproducible, and within one batch transducers can be manufactured with very small deviations between transducers in the same batch. This means that, with the claimed method, it is not necessary to measure the actual diaphragm tension on each individual transducer before adjusting the tension. It suffices to measure the actual diaphragm tension on selected transducers on selected wafers in the batch, and with sufficiently precise and predictable processes it is even not necessary to measure the actual diaphragm tension of transducers in every batch.
The resulting diaphragms can be applied in many types of transducers such as condenser and other microphones, and specifically, in micromachined microphones based on semiconductor technology, in microphones in battery-operated equipment, sensitive microphones, and microphones with a high signal-to-noise ratio.